Arizona Attorney General Thomas Horne talks with reporters outside the U.S. Supreme Court after oral arguments in the case challenging Arizona's voter registration law on Monday.

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The U.S. Supreme Court heard arguments Monday in a case that seeks to redefine a federal law aimed at streamlining the nation's voter registration process.

Congress enacted the law 20 years ago after it found that 40 percent of eligible voters were not registered to vote. Under the 1993 National Voter Registration Act, people can register by mail to vote in federal elections using a standard federal form. The form, among other things, asks prospective voters whether they are U.S. citizens and requires them to sign to the statement, under penalty of perjury.

At the time the federal law was passed, Arizona and half the other states had similar registration forms. But Arizona voters subsequently approved a referendum that requires further proof of citizenship — a birth certificate, naturalization form or passport, for instance. A federal appeals court struck down the Arizona law, saying that it conflicts with and is trumped by federal law. Arizona appealed to the Supreme Court, where the justices heard arguments Monday.

Opponents of the Arizona law argued that it is unnecessarily burdensome and improperly prevents U.S. citizens from voting. SevaPriya Barrier, a native of Arkansas, was one of those citizens affected by the Arizona law and was at the high court on Monday. She moved to Arizona shortly before the 2010 federal elections and tried to register to vote with the federal form. "At the time that I filled it out and submitted it, I did not have an Arizona driver's license yet. My registration was rejected for failure to provide that proof of citizenship," she said.

Barrier adds that she received the rejection notification after the deadline to register had passed.

"So I was denied that fundamental right to vote," she said.

But Arizona Attorney General Thomas Horne counters that the federal system does not provide a sufficient guarantee of citizenship, "because it's an honor system." The federal regime essentially says "if you sign and say you're a citizen, we have to believe you," he says.

Arizona's Arguments

Inside the courtroom, the justices gave both sides a very hard time, starting with Horne.

Justice Sonia Sotomayor interrupted Horne to ask the first question, going straight to the purported justification for the state proof-of-citizenship law: "Why would you think that Congress ... didn't consider the issue of fraud" and strike "the balance it wanted" when it enacted the federal law?

Justice Ruth Bader Ginsburg followed up with this observation: "Congress did specify how citizenship was to be handled" — Congress decided to use "a signed attestation subject to the penalty of perjury. So it's not as though the federal form" didn't deal with citizenship. It did, and Arizona's law is adding "something else."

Justice Elena Kagan pursued the point: Why have a federal form if Arizona could just say that, in addition, "you have to give 10 more items of information? Then the federal form just becomes another hoop to jump through."

Noting that the federal Election Assistance Commission is charged with approving any state-specific instructions to the federal form, Justice Antonin Scalia asked whether Arizona had asked to add on the proof-of-citizenship requirement to the federal form.

The state had asked for the change, but had been denied, replied Horne.

Scalia asked: Why didn't you go to court to challenge that?

"I don't know," replied Horne. That decision was made by "my predecessor."

Justice Anthony Kennedy, who could well cast the deciding vote in the case, wondered whether the state could also ask for proof of address or proof of date of birth on the form.

Horne replied that the state could add those requirements if the federal commission found them to be consistent with the federal law.

Kennedy went on to opine that the federal form "is not worth very much." And Horne pointed out that some states — for example, Louisiana — had been allowed to require a Social Security number or utility bill to prove residency.

"Yes, that's the kind of thing you should have had and that your predecessor should have asked for to be included in the federal form," Scalia said in response.

But under federal law, Horne argued, the states still have the ultimate responsibility to determine voter eligibility, and so the state should be allowed to require proof of citizenship beyond what is required in the federal form.

Kennedy, however, noted that those challenging the Arizona law would argue that the federal form is "presumptive evidence" of registration, unless the state has actual evidence to the contrary. "Otherwise, the whole utility of a single form is ... gone."

The Challengers' Arguments

Next up to the lectern was Patricia Millett, representing those challenging Arizona's proof-of-citizenship requirement. Arizona, she said, "simply disagrees with the balance that Congress drew." Under Arizona's proof-of-citizenship requirement, 31,000 people were rejected from voting. Of those, 11,000 subsequently registered, she said, but they had to "do the double gantlet that Congress was trying to eliminate."

Noting that the federal law requires states to "accept and use" the federal form, Justice Samuel Alito asked whether that means the state can make no further inquiry of the person.

In essence, Millett said yes, but she added that the state may check the voter's information against other databases in its files — for example, to make sure that a felon does not vote and to ascertain that the registrant lives at the address he gave.

Chief Justice John Roberts seemed dubious about the whole system and wondered how a state system could work separately for state and federal elections. He asked: Would there be two separate voter registration rolls?

In fact, every state has, in the interest of economy, hitched its wagon to the federal registration system. No state has separate systems for state and federal elections.

But some justices still seemed skeptical.

"This seems to me like a crazy system," Alito observed.

"It seems to me," said Kennedy, that when the lower court struck down Arizona's law it ignored "the proposition that the state has a very strong and vital interest in the integrity of its elections, even when those ... are elections of federal officials."